Transcript 1_Introduction - Department of Astronomy

Why is the Universe Lumpy?
Monday, November 24
The average density of the universe
is 10-26 kg/m3.
However, most of the universe is slightly
less dense than average (voids).
Some of the universe is much denser than
average (stars, white dwarfs, black holes…)
Dark energy: apparently uniform density,
with no lumps.
Dark matter: large lumps, about 1
megaparsec across.
Ordinary matter (protons, neutrons,
electrons): small, but very dense, lumps.
As we have seen, gravity tends to increase
the lumpiness of matter.
1
2
3
4
The dense regions present when the
universe became transparent have evolved
tbecome clusters & superclusters today.
However, gravity alone can’t account for the
extreme lumpiness of ordinary matter.
500,000 parsecs
50,000 parsecs
Dark
halo
Luminous
galaxy
Luminous part of a galaxy
(electrons, protons, & neutrons)
is much smaller than the dark part
(Weakly Interacting Massive Particles).
What’s special about
electrons, protons, & neutrons
that concentrates them at the
center of dark halos?
“Tootsie pop” hypothesis:
central luminous galaxy forms 1st,
then is “dipped” in dark matter.
“Twinkie” hypothesis: outer
dark halo forms 1st, then
luminous galaxy is “injected”.
Consider a gas of electrons, protons, helium
nuclei, and WIMPs all mixed together…
w
e
p
e
p
e
p n
n p
w
p
w
e
e
… and all moving in random directions.
e
w
p
Initially, the particles move rapidly.
They have a high temperature…
w
e
p
e
p
e
p n
n p
w
p
w
e
e
… and therefore a high pressure.
e
w
p
However, the ordinary particles emit
photons, which carry away energy…
w
e
p
e
p
e
p n
n p
w
p
w
e
e
…so ordinary particles (but not WIMPs)
slow down.
.p
e
w
Ordinary particles, no longer supported by
pressure, flow where gravity takes them…
w
e
p
e
p
e
p n
n p
w
p
w
e
e
…to the densest clumps of dark matter
e
w
p
Astronomy jargon:
“falling down the gravity well.”
Since ordinary stuff,
made of electrons, protons, & neutrons,
can easily dump its excess energy,
it falls toward dense regions.
Modified Twinkie Hypothesis:
originally (dark matter) sponge cake &
(ordinary matter) creme filling coexist.
Gravity “injects” the ordinary matter to
the center of the dark matter.
Galaxies form because ordinary matter
cools down (by emitting photons) and falls
to the center of dark halos.
Why do galaxies curdle into tiny stars,
instead of remaining as homogenous gas
clouds?
Look at where stars are forming now.
In the Whirlpool Galaxy,
we see newly formed
stars in dense, cold
molecular clouds.
In regions where the gas is cooler and
denser than elsewhere, hydrogen forms
molecules (H2).
These cool, dense
regions are thus called
“molecular clouds”.
Consider a
small, dense
molecular cloud.
Mass = 1 Msun
Radius = 0.1 pc = 4,000,000 Rsun
Temperature = 10 Kelvin = Tsun/580
Molecular clouds are usually stable;
but if you hit them with a shock wave,
they start to collapse gravitationally.
Once the
collapse is
triggered, it
“snowballs”.
shock waves
Once gravity has reduced the radius
of the cloud by a factor of 4,000,000,
it’s the size of a star.
1

4,000,000
Why doesn’t the molecular cloud collapse
all the way to a black hole?
Escape speed from
molecular cloud ≈ 0.3 km/sec
Escape speed from
star ≈ 600 km/sec
Escape speed from
black hole = 300,000 km/sec
As the gas of the molecular cloud is
compressed, it becomes denser.
As the gas is compressed,
it also becomes hotter.
When the gas temperature is high
enough (T ≈ 10 million Kelvin), nuclear
fusion begins!
Nuclear fusion keeps the
central temperature and
pressure of the star at a
constant level.
The star is static (not
contracting or expanding)
because it’s in hydrostatic
equilibrium.
Hydrostatic equilibrium = a balance
between gravity and pressure.
Pressure increases as you
dive deeper into the ocean:
pressure increases as you
dive deeper into the Sun.
Gas flows from regions of high pressure to
regions of low pressure.
For gas in the Sun, pressure
creates a net outward force, gravity
creates a inward force.
The Sun is in hydrostatic equilibrium.
The Sun is like a fat guy
on an inflatable chair.
fusion
energy
pressure
gravity
What happens when nuclear
fusion ends inside a star?
Pressure drops: gravity compresses
star to a denser object.
Small stars → white dwarf
(very dense)
Larger stars → neutron star
(very, very dense)
Largest stars → black hole
(ultimate in density)
Wednesday’s Lecture:
The Inflationary Universe
Problem Set #7 due
Reading:
Chapter 12